https://phys.org/news/2019-01-extreme-cold-invades-climate.html

"It is a mass of very cold air that typically sits right on the North Pole and tends to be restricted to the North Pole by the jet stream," explained Ben Kirtman, professor of atmospheric sciences at the University of Miami Rosenstiel School of Marine and Atmospheric Science.

Normally, the jet stream—which is a strong a current in the atmosphere—would keep this cold air in the Arctic, but when the jet stream wavers, or weakens, the chill can spill.